Due to the rising costs and depleting reserves of fossil based oil, it is desired to replace petrochemicals with chemicals based on renewable resources. Most polymers in use today are based on petrochemical derived monomers. While there has been some activity to synthesize polymer materials using bio-based raw materials, in many cases the performance properties are inferior to that of the current petrochemical based technology. Thus, there is a need for new polymers based on renewable resources that have excellent performance properties.
Vegetable oil based materials have been used a long time in paints and varnishes and in alkyd resins. Vegetable oils are derived from the seeds of various plants and are chemically triglycerides of fatty acids. That is, vegetable oils consist of three moles of fatty acids esterified with one mole of glycerol. As shown below in Formula I, fatty acids are linear carboxylic acids having 4 to 28 carbons and may be saturated or ethylenically unsaturated.

Different plants produce oils having differing compositions in the fatty acid portion of the oil. Naturally-occurring vegetable oils are by definition mixtures of compounds, as are the fatty acids comprising them. They are usually either defined by their source (soybean, linseed, etc.) or by their fatty acid composition. A primary variable that differentiates one vegetable oil from another is the number of double bonds in the fatty acid; however, additional functional groups can be present such as hydroxyl groups in castor oil and epoxide groups in vernonia oil. Table 1 below identifies the typical fatty acid composition for some commonly occurring vegetable oils.
TABLE 1Fatty AcidUnsaturationCoconutCornSoybeanSafflowerSunflowerLinseedCastorTall Oil FATungC12Lauric044C14Myristic018C16Palmitic01113118116254C18Stearic0644364131Oleic1729251329227468Ricinoleic187Linoleic2254517552163414Linolenic39125233Eleaosteric380Iodine7.5-10.5103-128120-141140-150125-136155-20581-91165-170160-175Value
Sucrose, β-D-fructofuranosyl-α-D-glucopyranoside, is a disaccharide having eight hydroxyl groups. The combination of sucrose and vegetable oil fatty acids to yield sucrose esters of fatty acids (SEFA) as coating vehicles was first explored in the 1960s. Bobalek et al., Official Digest 453 (1961); Walsh et al., Div. Org. Coatings Plastic Chem. 21:125 (1961). However, in these early studies, the maximum degree of substitution (DS) was limited to about 7 of the available 8 hydroxyl groups. The resins do not appear to have been commercialized at that time. In the early 2000s, Proctor & Gamble (P&G) Chemicals developed an efficient process for industrially manufacturing SEFAs commercially under the brand name SEFOSE with a high DS of at least 7.7 (representing a mixture of sucrose hexa, hepta, and octaesters, with a minimum of 70% by weight octaester) (U.S. Pat. Nos. 6,995,232; 6,620,952; and 6,887,947), and introduced them with a focus on marketing to the lubricant and paint industries. Due to their low viscosities (300-400 mPa·s), SEFOSE sucrose esters can be used as binders and reactive diluents for air-drying high solids coatings. Formula II displays the possible molecular structure of a sucrose ester with full substitution. Procter and Gamble has reported a process to prepare highly substituted vegetable oil esters of sucrose using transesterification of sucrose with the methyl esters of sucrose (U.S. Pat. No. 6,995,232).

An epoxide group is a three-membered, cyclic ether containing two carbon atoms and one oxygen atom. An epoxide can also be called an oxirane. As in known in the art, an epoxy group has the structure shown in formula III in which R and R′ are organic moieties representing the remainder of the compound.

Epoxy resins are materials consisting of one or more epoxide groups. Due to the strained nature of the oxirane ring, epoxide groups are highly reactive and can be reacted with nucleophiles such as amines, alcohols, carboxylic acids. Thus, epoxy resins having two or more epoxy groups can be reacted with compounds having multiple nucleophilic groups to form highly crosslinked thermoset polymers. Oxiranes can also be homopolymerized. Epoxy resins having two or more epoxy groups can be homopolymerized to form highly crosslinked networks. Crosslinked epoxy resins are used in a large number of applications including coatings, adhesives, and composites, among others. The most commonly used epoxy resins are those made from reacting bisphenol-A with epichlorohydrin to yield difunctional epoxy resins.
Epoxidation of the double bonds in unsaturated vegetable oils results in compounds which incorporate the more reactive epoxy group. Epoxide groups, or oxirane groups, as discussed, can be synthesized by the oxidation of vinyl groups. Findley et al., J Am. Chem. Soc. 67:412-414 (1945), reported a method to convert the ethylenically unsaturated groups of triglyceride vegetable oils to epoxy groups, as shown in Scheme 1 below. A number of other processes and catalysts have been developed to also achieve epoxidized oils in good yields.

Generally, while there are four techniques that can be employed to produce epoxides from olefinic molecules (Mungroo et al., J. Am. Oil Chem. Soc. 85:887 (2008)), the in situ performic/peracetic acid (HCOOH or CH3COOH) process appears to be the most widely applied method to epoxidize fatty compounds. Scheme 2 displays the reaction mechanism, which consists of a first step of peroxyacid formation and a second step of double bond epoxidation. Recently, the kinetics of epoxidation of vegetable oils and the extent of side reactions was studied using an acidic ion exchange resin as catalyst and revealed that the reactions were first order with respect to the amount of double bonds and that side reactions were highly suppressed; the conversion of double bonds to epoxides was also high. Petrović et al., Eur. J. Lipid Sci. Technol. 104:293 (2002); and Goud et al., Chem. Eng. Sci. 62:4065 (2007). The catalyst, Amberlite IR 120, is an acidic ion exchange resin, a copolymer based on styrene (98 wt %) crosslinked by divinylbenzene (2 wt %). Its acidity is generated by sulfonic acid groups attached to the polymer skeleton.

Epoxides generated from the epoxidation of double bonds of ethylenically unsaturated fatty acids are known as internal epoxides—both carbons of the heterocyclic ring are substituted with another carbon. The most commonly used epoxy resins are the bisphenol-A diglycidyl ether resins. The epoxy groups on these resins are of the type known as external epoxides—three of the four substituent groups on the heterocyclic ring are hydrogen atoms. Since internal epoxides are much less reactive than external epoxides in most epoxy curing reactions, the roles traditionally assigned to epoxidized oils are as stabilizers and plasticizers for halogen-containing polymers (i.e., poly(vinyl chloride)) (Karmalm et al., Polym. Degrad. Stab. 94:2275 (2009); Fenollar et al., Eur. Polym. J. 45:2674 (2009); and Bueno-Ferrer et al., Polym. Degrad. Stab. 95:2207 (2010)), and reactive toughening agents for rigid thermosetting plastics (e.g., phenolic resins). See Miyagawa et al., Polym. Eng. Sci. 45:487 (2005). It has also been shown that epoxidized vegetable oils can be cured using cationic photopolymerization of epoxides to form coatings. See Crivello et al., Chem. Mater. 4:692 (1992); Thames et al., Surf. Coat. Technol. 115:208 (1999); Ortiz et al., Polymer 46:1535 (2005).
As noted, epoxidized vegetable oils have found use as plasticizers for polyvinyl chloride (PVC). When crosslinked directly using the epoxy groups, the resulting products are relatively soft due to the aliphatic nature of the vegetable oil backbone. Epoxidized vegetable oils have been further functionalized using acrylation, methacrylation, and hydroxylation.
Epoxy resins based on polyfunctional vegetable oil esters of sucrose can be crosslinked into high performance thermosets using cyclic anhydrides. See WO 2011/097484, the disclosure of which is incorporated herein by reference.
While the epoxy resin is 100% bio-based, the system uses petrochemical derived cyclic anhydride crosslinkers, which reduces the overall bio-based content of the thermosets. It is therefore of interest to use crosslinkers which are also bio-based to form thermosets that are 100% bio-based.
There are a large number of polyfunctional acids available, which are either currently available from bio-derived processes or for which bio-based processes are being derived. Some of these acids are shown in Table 2 below. These polyfunctional acids may be used as crosslinkers for vegetable oil-based epoxy resins, such as, for example, the epoxidized vegetable oil sucrose esters, since the acid groups are reactive with the epoxy groups and the functionality is two or greater.
TABLE 2Structures of exemplary bio-based acidsAcid NameCAS NumberStructureOxalic 114-62-7 Succinic 110-15-6 Pimelic 111-16-0 Suberic 505-48-6 Azelaic 123-99-9 Sebacic 111-20-6 Brassylic 505-52-2 Citric 77-92-9 Furan  Dicarboxylic acid 3238-40-2 Tartaric Acid 526-83-0
However, these acids are crystalline solids with high melting points, and it can be challenging to mix them with the epoxy resin and form a homogeneous mixture. Attempts at forming crosslinked materials by dispersing the diacids in the epoxy have resulted in materials with poor properties.
The reversible reaction of carboxylic acids with vinyl ether compounds leads to liquid, low viscosity materials, i.e., the carboxylic acids can be “blocked” via reactions with vinyl ether compounds. In the presence of the proper catalyst, the vinyl group can “deblock” from the carboxylic acid group and allow the acid to react with an epoxy group. See Nakane et al., Prog. Org. Coat. 31:113-120 (1997); Yamamoto et al., Prog. Org. Coat. 40:267-273 (2000), the disclosures of which are incorporated herein by reference. The blocking vinyl ether group can also be removed thermally.